The saga of Olympic cycling champion Tyler Hamilton's alleged positive test for blood doping and subsequent two-year ban has been given a name worthy of Sir Arthur Conan Doyle: the case of the vanishing twin.

It is a story now into its ninth month and the news last week that Hamilton's appeal against the verdict is being considered by the Tribunal of Arbitration in Sport means the arguments could run until September.

The hearing is expected to take place in Denver this month, but TAS has a four-month deadline before it delivers a ruling and thus a final verdict may come a full year after the American's positive test was first announced.

Last week, the cyclist was still maintaining that the positive is a dreadful mistake, which has cost him more than a million dollars in lost earnings and legal fees. 'I am innocent and I believe in this process,' he said.

The former downhill skier is one of the toughest cyclists in the world, a man who rode through the 2003 Tour de France with a broken collarbone but managed to take fourth overall. The charge against him is that he attempted to improve his performance by injecting someone else's blood to boost his red cell count - and thus his endurance - shortly before winning a stage of the Tour of Spain on 11 September last year.

That sounds outlandish enough, but the New Englander's defence is one of the strangest ever made in an anti-doping case. It centres on the theory that Hamilton is one of twins, but that his twin died in utero, and before he or she did so, Hamilton received a small number of 'foreign' stem cells, producing subtly different red blood cells. These, say scientists who are defending the American, could explain the discovery of two types of blood in his system.

The 'vanishing twin' is one cause of a condition where a person has two types of blood, which is known as chimerism. It was brought to Hamilton's attention by Dr David Housman, a professor of molecular biology at the Massachussetts Institute of Technology, who read about the case in the sports pages and offered to testify on Hamilton's behalf.

'I read it and said "Wait a second. I don't think the explanation they give for the blood test is the only possible explanation",' Housman, who was quoted by Hamilton's defence at a US Arbitration panel hearing, told the New York Times.

Scientists are divided over how common chimerism actually is. Researchers such as Dr Ann Reed of the Mayo Clinic, who has published several studies of the phenomenon, claim that it may affect up to 70 per cent of the population and could be the cause of phenomena such as donor bone marrow that apparently matches a recipient but is rejected.

Housman has rubbished the scientific credibility of the International Olympic Committee's test for blood doping, claiming that the testimony of Dr Ross Brown, a haematologist from the Royal Prince Alfred Hospital in Sydney, 'is riddled with factual errors and inconsistencies'.

Brown has examined 20,000 blood samples and not one showed signs of chimerism. 'The question is "Have you ever looked?"' said Hamilton's wife, Haven, in an interview.

Housman added that what made the issue of identifying chimerism doubly fraught was that it could effectively turn on and off the production of foreign blood cells.

Hamilton's defence has already been thrown out by a US arbitration panel and other facts will count against him. In June 2004, he received a formal warning from the International Cycling Union that random blood tests 'showed signs of manipulation'.

Most telling, however, is the fact that the test for traces of foreign blood cells has been in clinical use for a decade. It was originally developed to determine whether a foetus is swapping blood cells with a mother who has an incompatible blood type, which can lead to the foetus being harmed. The method is so sensitive that it can detect a handful of 'foreign' blood cells in a given sample.

According to Michael Ashenden, the Australian physiologist who headed the research group which devised the test, there is no room for doubt: 'You don't make a mistake with this. It's always negative or positive.'